Mixture of THP and a moderator to tan leather

ABSTRACT

A mixture of THP and a moderator is used to tan leather. The moderator is selected from metaphosphates and polyhydroxy or polypyrolidyl compounds such as glycerol, carboxydrates PVA or PVP.

This application is a U.S. National Phase Application under 35 USC 371of International Application PCT/EP00/05471 (published in English) filedJun. 14, 2000 now WO/79001 A1.

The present invention relates to a novel process for tanning leather.

Tanning is a process for the preservation of skins, by which is meantthe collagen-containing integuments of vertebrates including mammals(e.g. cows, pigs, deer, goats, sheep, seals, antelope, mink, stoats andcamels), fish, (e.g. sharks), reptiles (e.g. snakes, lizards andcrocodiles), and birds (e.g. ostrich). Skins comprise a layer ofcollagen, and tanning entails reacting the collagen with a crosslinking, or tanning, agent to cross link reactive sites within thecollagen molecule. The product of the cross linking is leather, which issubstantially less susceptible than skin to bacterial degradation.

A consequence of the cross linking is an increase in the minimumtemperature at which the wet leather tends to shrink. This shrinktemperature is often used as an indication of the degree of tanning.

The collagen layer of the skin is typically separated from fats,connective tissue and other subcutaneous protein, and optionally fromthe outer keratinous layer, by a combination of chemical and physicalsteps. The former may include liming, bating, pickling and/ordegreasing.

The skin is then subjected to treatment in one or more stages withvarious tanning agents selected to give the desired end properties.

The main types of tannage are: vegetable tannage, based on tannin as theactive cross linking agent; mineral tannage using various polyvalentmetal salts, especially salts of chromium, aluminium, iron, orzirconium; and synthetic tanning, agents, referred to as “syntans”.Syntans include replacement syntans which are active tanning agentscapable of tanning leather when used as the sole tannage e.g. byreacting with collagen at two or more sites to form cross links, andauxiliary syntans which are added to other tannages to modify thecharacter of the leather but which are not in themselves active tanningagents. Auxiliary syntans are absorbed by the leather or may react withcollagen at one site only. Syntans include various polymers andcopolymers, such as those obtained by condensing formaldehyde with, forexample, phenols and/or aryl sulphonates, and acrylate, methacrylate,acrylamide and/or acrylonitrile homopolymers and copolymers.Formaldehyde itself and dialdehydes such as glutaraldehyde are also usedin tanning, usually in combination with other tannages.

For centuries the production of leather was based on the vegetabletannages which produce the characteristic brown colour traditionallyassociated with leather. One of the first mineral tannages was alum, butcurrently the most widely used tanning agent is chrome, usually in theform of basic chromium sulphate, which produces a blue grey leather withhigh shrink temperatures. However mineral tannages in general, andchrome tannages in particular are under pressure on environmentalgrounds. Syntans are less environmentally harmful than mineral tanningagents.

Tetrakis (hydroxymethyl) phosphonium salts which will be referred toherein as “THP salts” have long been used as fire-retardants fortextiles and have been applied to the keratinous (fur) side of skins forthis purpose. The salts may be applied directly to the fabric or in theform of precondensates which are water soluble or sparingly watersoluble copolymers of THP with organic nitrogen compounds such as ureaor an amine and which are referred to herein as “THP condensates”. THPsalts have also been known as possible ingredients of tanning liquorsfor more than thirty six years. U.S. Pat. No. 2,992,879 referred to THPchloride (THPC) as an unsatisfactory tanning agent on its own, andrecommended a combination of THPC and a phenol such as resorcinolspeculating that the two react together to form an effective tanningagent when the pH is raised. In fact THP salts do copolymerise withphenols such as resorcinol (see, for example, Textile Research Journal,December 1982, P743). U.S. Pat. No. 3,104,151 describes the use of suchTHPC phenol copolymers as pretannages for leather in which the maintannage is vegetable or mineral. GB 2 287 953 describes the use of THPsalts as cross linkers in conjunction with melamine formaldehyde or ureaformaldehyde prepolymers, in order to form a copolymeric tanning agentin situ in the tanning liquor. EP 0 559 867 describes the use ofphosphonium salts such as THP sulphate (THPS) on raw or cured skin priorto tanning e.g. in acid degreasing. EP 0 681 030 describes the use ofTHPS as a cross linker for casein finishes applied to leather aftertanning. GB 2 314 342 describes the use of hydroxyalkyl phosphines andphosphonium salts as tanning agents in conjunction with aromatic anionicsyntans and EP 0 808 908 describes the use of THP salts with condensablenitrogen compounds.

THP salts are stable under acidic conditions in the absence of air oroxidising agents. At pH above 3 and in the absence of oxidising agentsthey are gradually converted to the parent base,tris(hydroxymethyl)phosphine commonly referred to as THP. Conversion israpid and substantially complete between pH of about 4 and 6. Above pH7, or in the presence of oxidising agents THP salts or THP are convertedto tris(hydroxymethyl)phosphine oxide (THPO), conversion being rapid andsubstantially complete at pH above about 10, e.g. 12. It has beenstated, e.g. in U.S. Pat. No. 2,993,744, that THPO is the effectivetanning agent in THP based tannages.

Contrary to statements in the art, THPO is not effective as a tanningagent for leather, and THP salts are also ineffective as tannages.Moreover THP used in conjunction with co-condensable monomers orpolymers provides complex systems which are difficult to control toobtain consistent results.

WO99/23261 describes the use of THP on its own as an effective maintanning agent. THP is usually most effective when formed in situ byfirst impregnating the leather with a THP salt, in the substantialabsence of monomers or prepolymers which react or copolymerise with THPand raising the pH above 4 and preferably above 5. WO99/23261 alsodescribes the use of THP condensates as main tanning agents.

In addition to THP, compounds of the formula R P(CH₂OH)₂, referred toherein as “THP analogues”, where R is an organic group which does notreact chemically with collagen, such as a C₁₋₂₀ alkyl, alkenyl, aryl,aralkyl, alkaryl, polyalkyleneoxy, alkylpolyalkyleneoxy orpolyalkyleneoxy alkyl group have been referred to in the literature ashighly effective tanning agents.

For convenience “THP” will be used herein, where the context permits, torefer generically to THP, THP salts, THP condensates and THP analogues.A major problem when tanning with derivatives of formaldehyde, includingformaldehyde condensate syntans and THP is the evolution of formaldehydeduring tanning and the presence of formaldehyde residues in the leatherwhich give rise to objectionable odours and may cause a health hazard toprocess operators.

The steps required to produce leather including the pickling anddegreasing which usually precede tanning, remove most of the naturaloils and fats from leather. These are normally at least partiallyreplaced after tanning by fat liquoring, which entails contacting theleather with an aqueous emulsion of oils and fats which soften andlubricate the finished leather. It has been found that leather which hasbeen tanned with THP alone is tight and tends to be relatively hard. Itrequires high levels of fat liquor to soften it. It would be useful tobe able to obtain the advantages of THP, but prepare a fuller, softerand more easily fat liquored leather.

It is often necessary to split, e.g. bovine leather, prior to retanning,in order to obtain the desired thickness. Conventionally tanned leathermay split unevenly and the split leather may require substantial shavingto obtain a consistent thickness.

A further problem is swelling of the skin, control of which usuallyrequires the presence of salts such as chloride or sulphate.

One object of the present invention is to provide tannages which exhibitthe advantages of THP but provide leather which is fuller, softer andrequires less fat liquor than leather tanned with THP alone.

A further object is to provide leather with lower formaldehyde levelsduring processing and on the finished leather, than are common intannages based on formaldehyde derivatives. Another object is to provideleather which gives a more even split than conventionally tannedleather.

A further object is to obtain leather with reduced tendency to swell,compared with normal THP tannage.

We have now discovered that mixtures of THP with certain moderators,which appear to interact with THP to mask its OH groups at leastpartially, can be applied to skins to provide full soft leather which isreadily fat liquored, contains reduced formaldehyde levels, is moreeasily split and shaved and exhibits reduced swelling tendencies.

We have found for example that a mixture of THP with a water solublemetaphosphate or with a water soluble polyhydroxy or polypyrolidonecompound containing at least three adjacent, geminal or vicinal groupsor pyrolidone groups provides improved tanning compared with THP alone,and in particular provides a soft, less tight and more easily fatliquored product.

The leather also shows reduced tendency to swelling which may becontrolled without the use of salts.

In addition formaldehyde levels, in the process and in the finishedleather are substantially less using the aforesaid mixtures than areobserved when THP is used alone.

It has also been found that leather tanned using the mixture is moreeasily shaved and split.

We believe that THP interacts chemically with the polyhydroxy compoundspossibly by hydrogen bonding, and that this moderates the reaction withcollagen.

Our invention therefore provides a mixture of THP as herein defined withat least one water soluble moderator selected from (A) metaphosphatesalts and (B) aliphatic polyhydroxy or polypyrrolidone compoundscomprising at least three adjacent, geminal or vicinal

groups where R is hydrogen or an alkyl group, and X is a hydroxyl orpyrrolidone group.

Our invention also provides a method of tanning or degreasing leatherwhich comprises contacting collagen with a mixture as aforesaid.

Preferably the water soluble polyhydroxy or polypyrrolidone compound hasa molecular weight less than 200,000, more preferably less than 150,000,most preferably less than 100,000, e.g. less than 75,000. We especiallyprefer compounds with a mole weight less than 50,000.

The polyhydroxy compounds preferably comprise hydroxy methylene groupswhich may be adjacent (i.e. directly linked as in glycerol), geminal(i.e. linked to the same carbon atom as in pentaerythritol) or vicinal(i.e. linked to adjacent carbon atoms, as in trimethylolpropane). Thusthe polyhydroxy compounds useful in the invention comprise compounds ofthe formulae:

wherein each n may independently be 0, 1 or 2 and each R isindependently hydrogen or an alkyl, a hydroxy- or polyhydroxy- alkyl, acarbonyl, a carboxyl, a C₁₋₂₅ acyl, a carbonyl- carboxy- or C₁₋₂₅ acyl-substituted alkyl group or an alkyl group or hydroxy-, carboxy-carbonyl- or C₁₋₂₅ acyl substituted alkyl group which is interrupted byone or more ether or carbonyl linkages or wherein two or more R groupsmay together constitute one or more alicyclic rings, optionallysubstituted with one or more hydroxy, carbonyl, carboxy and/or acylgroups and/or interrupted by one or more ether or carbonyl linkages.

The aliphatic polyhydroxy compound may for example be glycerol,sorbitol, trimethylol propane, pentaerythritol, 1,2,3-trihydroxy butane,1,2,3,4-tetrahydroxy butane, 1,2,3-trihydroxy pentane, 2,3,4-trihydroxypentane, tetrahydroxy pentane, pentahydroxy pentane, water solublecarbohydrates including mono and disaccharides and soluble starches andalginates. Examples of suitable carbohydrates include a sucrose,glucose, maltose, lactose, dextrose, fructose, xylose, arabinose,mannose, ribose and rhamnose and dialdehyde starch. The compound mayalso be a derivative of a carbohydrate including alkyl glycosides suchas a C₁₋₂₅ alkyl glucoside or polyglucoside, or ascorbic, mannonic orgluconic acids, or alginates or sorbitol or mannitol or C₁₋₂₅ acylesters of either sucrose or sorbitan. Generally we prefer not to usenitrogen containing derivatives.

A preferred group of polyhydroxy compounds comprises polyvinyl alcoholsand, in particular, relatively low molecular weight polyvinyl alcohols.We prefer PVAs with a mean molecular weight less than 70,000, especiallyless than 50,000, e.g. less than 30,000.

Also highly effective in modifying the action of THP tannages ispolyvinyl pyrrolidone which appears to function in an analogous mannerto polyvinyl alcohol.

THP condensates containing high proportions of nitrogen andcorrespondingly low proportions of uncondensed THP, e.g. less than 4:1molar THP:nitrogen compound are generally less effective withpolyhydroxy or polypyrolidone compounds than the uncondensed THP.However THP condensed with lower proportions of nitrogen compound e.g.condensates with greater than 4:1, especially greater than 5:1, forinstance 5:1 to 7:1 molar THP/nitrogen compound, which contain some freeTHP, give particularly good results.

The metaphosphate salt is a cyclic condensed phosphate having theformula (MPO₃)_(n) where M is preferably alkali metal or ammonia, e.g.potassium or most preferably sodium and n is from 2 to 10, preferably 3to 8. e.g. 5 to 7. Particularly preferred is sodium hexametaphosphate.In principle any water soluble metaphosphate salt of a cation which doesnot react with THP may be used.

The relative proportion of THP and moderator may be equimolar or maycomprise an excess of either component. Preferably the THP is in anexcess of up to 20:1 molar, depending on the characteristics required.Generally the higher the proportion of polyhydroxy compound the lesstight the leather. Proportions between 10:1 and 2:1 molar are generallypreferred.

The THP and moderator may be supplied as or part of a formulatedproduct, premixed or mixed in situ in the tanning operation.

The THP may be used in conjunction with syntans, e.g. by treating theskin with said THP and a syntan (preferably a syntan which does notreact with THP under normal tanning conditions) and/or by treating theskin in a plurality of stages at least one of which entails treatmentwith syntan and at least one other of which entails treatment with theTHP.

For the purpose of this specification “syntan” is used to refer toreplacement syntans which are synthetic organic compounds capable ofreacting with collagen at two or more sites to form cross links and alsoto auxiliary syntans which do not in themselves contribute substantiallyto the cross linking but which are physically absorbed by the leather orreact at no more than one site so as to modify the physical propertiesof the leather. For example the term includes any water soluble polymerprepared by copolymerising formaldehyde, which is capable of increasingthe shrink resistance of collagen and which comprises at least two unitsof the formula

where each M is an aryl group such as a phenyl, naphthyl or anilinegroup substituted with one or more hydroxyl and/or sulphate, sulphone orsulphonimide groups or a urea or melamine residue. As used herein theterm “syntan” also includes resin syntans which are homopolymers andcopolymers of unsaturated carboxylic acids or their salts, esters,amides or nitrites, e.g. acrylic acid, methacrylic acid, acrylamide,acrylonitrile, maleic acid, fumaric acid, itaconic acid, aconitic acid,crotonic acid, isocrotonic acid, citraconic acid, mesaconic acid,angelic acid, tiglic acid and cinnamic acid. The copolymers may alsocomprise other vinylic comonomers such as styrene. Also included areacetone condensates with, for example sulphones and sulphonamides. Resinsyntan may modify such properties as dye levelling, filling, grainappearance, break, grain strength, buffing nap, softness and tensile ortear strength.

The THP tends to react with nitrogen containing syntans and with othernitrogenous compounds and also to some extent with aromatic syntans.Such reactions may compete with cross linking reactions of both the THPand the syntan resulting in chaotic behaviour when the two are usedtogether. Such systems are so highly sensitive to the conditions of usethat it may be difficult to control them so as to obtain the consistentresults required for a viable commercial process.

For these reasons we prefer not to use mixtures with such compounds.However it is possible to precondense the THP with, for example,co-condensable nitrogenous compounds such as urea, melamine,dicyanodiamide and/or aliphatic amines to form condensates comprisingtwo or more phosphorus atoms and having at least two hydroxymethylgroups.

High pH is preferably avoided because it converts the phosphine group tophosphine oxide, which is substantially ineffective as a tanning agent.We prefer that solutions for use according to our invention should havea pH less than 10, more preferably less than 9, especially less than 8,typically less than 7.5, most preferably 4.5 to 7. We prefer that theTHP contains less than 15%, more preferably less than 10%, e.g. lessthan 9% of THPO based on the weight thereof. Generally the less THPOpresent the better.

The THP is preferably used in a total concentration of from 0.01 to 20%by weight based on the total weight of the tanning liquor, morepreferably 0.5 to 10%, e.g. 1 to 5%, most preferably 1.5 to 4%. Thetotal proportion of the THP used is preferably from 0.3 to 20% by weightbased on the weight of wet skin, more preferably 1 to 15%, especially1.5 to 10%, most preferably 2 to 5%.

We particularly prefer processes in which skins are treated with anauxiliary syntan prior to tanning with the THP.

When the mixture is used in combination with a syntan, the latter ispreferably a polyacrylate, polymethacrylate, or copolymer of acrylicand/or methacrylic acid with acrylonitrile and/or acrylamide. Typicallythe polymer has a molecular weight in the range 1,000 to 200,000, moreusually 3,000 to 100,000.

The syntan is preferably present at a concentration of from 0.5 to 35%by weight of the tanning liquor. e.g. 1 to 20%, more preferably 2 to 10%especially 3 to 6%. The total proportion of syntan used is preferablyfrom 1 to 20% by weight based on the wet weight of skins, e.g. 2 to 10%especially 3 to 5%.

The proportion by weight of THP to syntan may typically be from 1:10 to10:1, preferably 1:5 to 2:1, especially 1:2 to 1:1. The total proportionof tannages used is preferably from 2 to 20% active weight based on thewet weight of skins, e.g. 3 to 10%, especially 4 to 8%. The totaltannage used preferably comprises more than 80% by weight, morepreferably more than 90% by weight, e.g. more than 95% of the THP andsyntan. Where white leather is required, we prefer that the totaltannages consist essentially of the THP and syntan. In particular weprefer the leather is not tanned with vegetable or mineral tannage.

The THP and polyhydroxy moderator may be used in conjunction withmineral tannages for example, a tannage containing a mixture of THP,chrome and moderator gives strongly synergistic tanning with high shrinktemperatures and good area yield using only small amounts of chrome.Combinations of THP, moderator and aluminium salts are particularlypreferred, especially aluminium triformate. The latter givesparticularly high shrink temperatures. Alternatively the THP tannage maybe used as a pretan or re-tan in a mineral tanning operation

The THP is preferably applied in the substantial absence of monomers orprepolymers capable of copolymerising with the THP such as phenol, urea,melamine or their precondensates with formaldehyde. For the purposes ofthis specification, “the substantial absence of monomers or prepolymers”means less than the minimum that would be capable of reacting orcopolymerising with 50% of the THP, more preferably less than theminimum that would be required to react or copolymerise with 20%, e.g.less than 5% by weight based on the weight of THP, most preferably lessthan 2%, especially less than 1%.

The THP may be applied as a first tanning step with a syntan as a retan.Preferably the THP is applied to acidified skins following aqueousdegreasing. E.g. the initial pH is typically below 5, e.g. below 4. Weprefer that the pH be raised above 5 and preferably maintained above 6for the main duration of the tanning process.

The skins are preferably agitated in the tanning liquor for a sufficienttime to raise the shrink temperature above 75° C., more preferably above80° C., most preferably above 85° C.

According to a particularly preferred embodiment the polyhydroxycompound is a surfactant such as a C₆₋₂₅ alkyl polyglycoside, sucroseester or sorbitan ester and the THP and polyhydroxy compound are appliedto undegreased or partially degreased skins, in order to effectdegreasing or further degreasing thereof.

The tanned skins are typically washed with warm water and fat liquoredusing a suitable oil or blend of oils. Fat liquoring is normally carriedout after dyeing.

The invention will be illustrated by the following examples.

EXAMPLE 1

60 g pickled bovine skin was rotated for 10 minutes with 15% based onthe wet weight of skin of 8% sodium chloride solution. 3% based on thewet weigh of skin of 75% by wt. THP solution and 1% sucrose was addedand the mixture rotated for 3 hours. The solution was basified to pH 6.5by adding 2.25% based on the wet weight of skin of sodium bicarbonateand the skins left to rotate overnight.

After the first 3 hours penetration was tested with sodium selenitesolution. A pale, even colour indicated even penetration throughout theskin. In contrast a control using THPS without sugar gave only surfaceaction.

After basification a further sample was tested with sodium selenitesolution. A strong orange colouration evenly distributed was observedthroughout the skin. THP alone gave an uneven colouration. The examplewas repeated three times using respectively maltose, lactose andD-glucose instead of sucrose, all with similar results.

All the samples gave similar shrink temperatures within the range 81-83°C. except for the lactose which gave a shrink temperature of 78° C., butthe THP control felt hard and boardy while the other samples were softand pliable. All the examples of the invention 1 to 12 described hereinshowed a reduction in formaldehyde level of between 30 and 45% comparedwith the control.

EXAMPLE 2

400 g of brine pickled pelt was rotated for 10 minutes with 150% byweight of water based on the wet weight of the skins at pH 4.6. 3% THPbased on the wet weight of skins was added as a 75% aqueous solutiontogether with 3%, based on the wet weight of skins, of glycerol. After 3hours rotation at 35° C. the pH was 3.38 and the shrink temperature was58° C. After basifying to pH 4.2 with sodium bicarbonate the shrinktemperature was 64° C. Basification to pH 5.5 gave a shrink temperatureof 68° C. Further basification to pH 6.5 gave a shrink temperature of77° C.

Testing with sodium selenite showed even penetration and the product wassoft and pliable.

EXAMPLE 3

200 gm bovine pickled pelt was rotated overnight with a mixture of 3%based on the wet weight of skin of 75% wt/wt aqueous THPS and 3% basedon the wet weight of skin of a 30,000 to 70,000 mole wt polyvinylalcohol at a pH of 3.5. The pH was then raised to 6.5 with sodiumcarbonate. After a further hour's rotation the product was full and softwith a shrink temperature of 79° C.

EXAMPLE 4

Example 3 was repeated using a polyvinyl alcohol of molecular weightgreater than 70,000. The product was soft but less full than that ofExample 3. The shrink temperature was 75° C.

EXAMPLE 5

Example 3 was repeated using a 6:1 molar THPS/urea condensate instead ofTHPS. The product was soft and fill. The shrink temperature was 78° C.

EXAMPLE 6

Example 3 was repeated using a 3:1 molar THPS/urea condensate. Theproduct was full but less soft than Example 3. The shrink temperaturewas 76° C.

EXAMPLE 7

Example 6 was repeated using a PVA with a molecular weight greater than70,000. The product was less tight and hard than that obtained using theurea/THP condensate alone, but not as full as that of Example 6. Theshrink temperature was 75° C.

EXAMPLE 8

Example 3 was repeated using polyvinylpyrolidone of mole weightapproximately 10,000 in place of PVA. The product was full and soft andhad a shrink temperature of 77° C.

EXAMPLE 9

Example 3 was repeated using a 2:1 molar THPS/urea condensate instead ofTHPS. The product was soft and full, with little residual odour.

EXAMPLE 10

Example 3 was repeated using dialdehyde starch in place of PVA. Theproduct was full and soft.

EXAMPLE 11

60 gm pickled bovine hide was rotated for 10 mins with 150% floatcontaining 8% sodium chloride. 3% THPS/urea condensate (6:1 mole ratio)and 1% dextrose were added and the rotation continued for 3 hours. Smalladditions of sodium carbonate were made until the pH was 6.5.

Selenium indicator showed good penetration at the end of the three hourrotation. The shrink temperature was 75° C.

The leather was split and shaved prior to retanning with an acrylicresin sytan. The leather gave a more even split than a control withoutthe moderator, and required less shaving.

The retanned leather was washed and fat liquored using 10% by weight ofwet skin of a mixed sulphated/sulphited oil fat liquor, and finallydried.

The leather was a very full and soft white leather.

EXAMPLE 12

100 gm bovine skins and 150% water based on wet weight of skin wererotated for 10 minutes with 8% sodium chloride based on wet weight ofskin.

3% (based on the wet weight of skin) of a 76% aqueous THPS solution and3% of sodium hexametaphosphate were rotated for 3 hours at pH 2 to 4 andthen basified to pH 6.5.

The product was full and soft with a shrink temperature of 78° C.

What is claimed is:
 1. A mixture for use in tanning leather, saidmixture comprising: (I) a phosphorus-containing compound selected fromthe group consisting of tris(hydroxymethyl)phosphine (THP);tetrakis(hydroxymethyl)phosphonium salts (THP⁺ salts); condensates ofTHP with organic nitrogen compounds (THP condensates); and compounds ofthe formula RP(CH₂OH)₃, wherein R is an organic group which does notreact chemically with collagen in said leather (THP analogues); and (II)at least one water-soluble moderator selected from the group consistingof (A) metaphosphate salts and (B) aliphatic polyhydroxy orpolypyrrolidone compounds selected from the group consisting ofsorbitol, glycerol, trimethylolpropane, pentaerythritol, mannitol, monoand di-saccharide sugars, alginates and polyvinyl pyrrolidone.
 2. Themixture of claim 1, wherein said phosphorous containing compoundconsists essentially of a condensate of from 2 to 6 moles THP with 1mole urea.
 3. The mixture of claim 1, wherein said at least onewater-soluble moderator is selected from the group consisting ofsorbitol, glycerol, trimethylolpropane, pentaerythritol, mannitol, monoand di-saccharide sugars, alginates and polyvinyl pyrrolidone.
 4. Amethod of tanning leather which comprises contacting the collagen insaid leather with a tannage comprising the mixture of claim
 1. 5. Themethod of claim 4, further comprising tanning said leather with a syntanand/or a mineral tannage and wherein said mixture is contacted with thecollagen in said leather prior to, simultaneously with, or after tanningof said leather with the syntan and/or mineral tannage.
 6. The method ofclaim 4, wherein said leather is fat liquored subsequent to saidtanning.
 7. The method of claim 4, wherein said leather is shaved and/orsplit subsequent to said tanning.
 8. The mixture of claim 1 wherein thewater-soluble moderator is selected from the group consisting of saidmetaphosphate salts.
 9. The mixture of claim 1 comprising THP andmoderator in a relative proportion ranging from equimolar amounts up toan excess of THP in a 20:1 molar ratio.
 10. The mixture of claim 9wherein the relative proportion of THP to the moderator is between 10:1and 2:1 molar.
 11. The mixture of claim 9 wherein the water solublemoderator is the (A) metaphosphate salts.
 12. The mixture of claim 1wherein the water soluble moderator is said polyhydroxy compounds. 13.The mixture of claim 1 wherein the water soluble moderator is saidpolypyrrolidone compounds.